I am not going to speculate who would have won, but here are some very clear facts:
1 - Costa is bigger and stronger at around 215 lbs with a very solid Judo/BJJ foundation. His TDD is impeccable. Khamzat cannot power through Costa like he did with Usman. Costa will definitely out-power him in grappling exchanges. Usman was 190 lbs (while Khamzat was around 210 lbs) so completely apples to oranges drawing conclusions about Costa from tonight's fight.
2 - He fought Luke Rockhold at something like 10,000 feet elevation so anyone claiming he gasses out in 3 rounds without considering he fought at very high altitude, is totally ignorant of the facts.
3 -Costa was extremely well prepared for the fight (Usman was not with only 9 day notice) training at Khabib's gym and Gracie's gym in UAE for 40 days. He was already cutting weight down to 210 lbs about 2 weeks out from the fight.